This is the right track. But be aware that marign-left/right, border-radius, and padding are CSS attributes for HTML elements. The flyout and its buttons are SVG elements. SVG elements have a mostly different set of CSS properties. So 'fill' will work fine (that's SVG) as will 'cursor' (that's both SVG and HTML).
However, I don't think there's a way to modify radius and location on SVG elements using CSS.
Blake just added a feature to the next release which will allow you to set the radius of these buttons by setting a class variable: